Resolve "Geo: Statement timeout counting local job artifacts"

What does this MR do?

This MR adds an index to the file_store field on the ci_job_artifacts table.

Are there points in the code the reviewer needs to double check?

N/A

Why was this MR needed?

It is believed https://sentry.gitlap.com/gitlab/gitlabcom/issues/163635/ was caused by a missing index.

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [-] Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #45476 (closed)